This invention relates to methods for producing a slide surface on a light metal alloy and also to reciprocating piston engines such as internal combustion engines having cylinder linings with slide surfaces.
German Offenlegungsschrift No. 44 40 713 discloses a method of producing slide surfaces on cast iron parts consisting of several steps in which lubrication pockets are exposed in a slide surface so as to provide hydrodynamic lubrication in operation. These steps include machining of the slide surfaces and then processing the surfaces with a chemically and electrochemically inactive liquid applied under a pressure suitable for flake removal. By a combination of liquid erosion and frictional smoothing the lubrication pockets are exposed in the slide surface, forming in the aggregate a system of pressure microchambers to provide the requisite hydrodynamic lubrication. These pockets are produced by a honing operation which extracts small titanium carbide and nitride particles from the surface and the craters so formed are closed by further treatment. Liquid erosion and frictional smoothing expose these pockets again.
U.S. Pat. No. 5,080,056 discloses a process in which a substantially pore-free aluminum bronze alloy layer is applied by high velocity flame spraying to parts made of a magnesium alloy, the thickness of the applied alloy layer being then reduced by honing from about 1 mm to a final dimension of about 127 microns.
In contradistinction to the formation of a system of pressure microchambers, it is known that a communicating groove system may be obtained, for example on the surfaces of cylinder liners for internal combustion engines by a honing operation. Such honing produces intersecting scores which axe interconnected at the points of intersection and constitute an overall open system. A disadvantage of this system is that the part sliding on the slide surface, for example a piston ring on a piston of an internal combustion engine, will push oil contained in the scores ahead of it, preventing any significant build-up of hydrodynamic oil pressure on the slide surface. Consequently, along the edges of the scores there will be mixed friction between the two materials. The mixed friction lubrication systems which are in widespread use for surfaces of gray iron castings, however, are not applicable to slide surfaces on parts made of a magnesium alloy for reasons of weight.
U.S. Pat. No. 2,588,422 discloses an aluminum engine block having cylinder liners which are formed by thermal spraying. These liners are built up in two layers on the untreated surface of the engine block, the top layer being a hard slide layer such as steel about 1 mm in thickness and the lower layer being a molybdenous interlayer about 50 microns in thickness. The interlayer, containing at least 60% molybdenum, does not constitute a slide layer, but is necessary in order to bind the hard slide layer to the aluminum block. Preferably, the uniting layer is made up of pure molybdenum. The slide layer is a layer of hard metal, as for example carbon steel, bronze or stainless steel, in which the steel may be an alloy containing nickel, chromium, vanadium or molybdenum for example. In principle, this two-layer structure provides a good slide layer, but the cost of the double coating is substantial.
British Published Patent Application No. 2,050,434 discloses various coatings obtained by thermal spraying and having thicknesses from 0.5 to 2 mm. These coatings are formed on steel or cast parts for internal combustion engines, as for example piston rings or cylinder linings. In this case it is found that coatings consisting of equal parts of powdered molybdenum and powdered carbon steel are considerably less abrasion-resistant than coatings containing only 0.5-4.5 wt. % of molybdenum with 20-97 wt. % of metal carbides and possibly iron or ferrous alloys. To bind these coatings to magnesium alloys, the process described above with respect to U.S. Pat. No. 2,588,422 must be used.
British Patent No. 1,478,287 describes a powdered mixture for plasma coating of steel parts or castings such as piston rings, cylinder blocks or cylinder linings to produce layers bearing a thickness of about 762-1270 microns. The powder is a mixture of molybdenum, boron and cast iron containing at least as much cast iron as molybdenum and the boron is usually present is amounts up to 3% of the sum of molybdenum and cast iron. Such coatings, as example 1 of British Published Patent Application No. 2,050,434 A shows, no longer meet present-day performance requirements.
Another coating for cylinder liners is disclosed in U.S. Pat. No. 3,620,137 in which a plasma coating containing nickel and chromium, boron, silicon and possibly also iron in addition to at least 65 wt. % molybdenum is described. This coating is intended for cylinder liners made of gray cast iron and exhibits very small pores of 0.1 to 2 microns and an over-all porosity of 15%, corresponding to the coatings described in British Application No. 2,050,434 and British Patent No. 1,478,287.
As described above, various plasma coatings are known for aluminum alloys. The application of the same coatings to magnesium alloys, however, is not possible in principle, because magnesium alloys have a coefficient of heat expansion about 1/3 greater than that of aluminum alloys. Thus, the ratio of the heat expansion coefficients of such plasma coatings to those of magnesium alloys is about 1:3, so that high stresses can be expected between the plasma sprayed layer and the magnesium alloy.